Comparative analysis of protein composition, digestion dynamics, and amino acid release between preterm infant formula and human milk: Implications for preterm infant formula optimization

To advance precision nutrition and optimize feeding strategies for preterm infants, it is essential to further clarify the differences and interrelations in protein composition and digestive behavior between infant formulas (IFs) and human milk (HM). In this study, an integrated approach combining protein compositional analysis, protein digestion profiling, and targeted amino acid metabolomics was employed to systematically investigate these differences from the perspective of “protein composition– in vitro digestion behavior–amino acid release patterns.” Our findings revealed that β-lactoglobulin (4–11 % of total protein in IFs, absent in HM) exhibited pronounced aggregation during digestion, markedly enhancing its resistance to peptic hydrolysis. Caseins, present at higher levels and in more mineralized forms in IFs (37–48 % vs. 9.5–11 % in HM), further impeded enzymatic cleavage. In contrast, HM samples—particularly colostrum without human milk fortifier (HMF)—underwent a more gradual proteolytic process and displayed a more balanced amino acid release profile. Excessive release of certain amino acids (e.g., lysine, histidine, phenylalanine) in specific IFs may induce acute nitrogen metabolic imbalance in preterm infants. Glutamine and glutamic acid emerged as major contributors to the divergent amino acid release patterns between IF and HM, with their reduced liberation potentially linked to calcium soap formation and peptide masking. Additionally, the incorporation of full-strength HMF into HM appeared to obscure arginine-rich cleavage sites, which may partially account for the increased lactoferrin excretion observed in mixed feeding low-weight infants. Enhancing casein phosphopeptide content or aligning protein composition more closely with that of HM in IFs may represent promising strategies to reduce the nutritional outcome disparities between formula-fed and breastfed preterm infants.